1. Field of the Invention
The invention relates to the processing of two dimensional signals representing animated images, so as to reduce the volume of data to be transmitted or to be stored for representing such images. It finds a particularly important application in the field of visiophone and video-conference installations which raise the problem of the transmission of an acceptable quality image with a low data flow rate, for example 64 kbits per second for transmission over telephone channel.
2. Prior Art
Numerous proposals have already been made for reducing the data rate required. Some of these proposals use spatial correlation inside the image. Among the techniques which may be considered, the discrete cosine transform in encoding loops seems of particular interest.
It has also been proposed to use time correlation between successive images. The solution usually used consists, instead of transmitting signals individually representing the successive pixels of the image, in transmitting the difference between the current image signal and a predictor which is an estimation of the signal at the picture point (pixel) considered.
The value of the signal representing the same pixel in the preceding frame has in particular been used as predictor of the pixel in the current image. But this inter-image coding method does not take into account the fact that appreciable portions of the image may be in overall movement.
More advantageous methods take movement into account, which implies estimating the movement in the different portions of the image. The movement estimation method which is most interesting is the so-called block matching technique. The image is divided into blocks having a predetermined size. For each block of the current image, the most similar block is sought in a window containing the block in the preceding image. For each block, the movement vector causing this most similar block to correspond to the block of the current image is computed and it is transmitted to the decoder; a device for evaluating such a movement vector is described in European No. 0 181 215.
It is for example possible to search, in blocks of 8*8 of the current image, for the closest block in a window of 23*23 pixels.
At first sight, it would seem that the computing power required in this case, corresponding to about 1.6.times.10.sup.9 operations per second for CIF (Common Intermediate Format) images, would require circuits of a complexity such that the solution is not acceptable.